This invention relates to means for switching and more particularly to switches which employ an electrically conductive liquid therein.
Various attempts have been made to provide switches which are fast operating, accurate, and very sensitive. Generally, these efforts have been directed to the area of miniaturization of mechanical switches. Attempts have been made to adapt such switch mechanisms to the keyboards of such devices as cash registers, desk calculators, or the like or to use them as sensing devices. All too often such switches suffer from one or a number of deficiencies and their use often requires a compromise in operating characteristics. Fast, miniaturized switches are believed to be often failure-prone and of a complex design. Larger, simpler switch constructions are, on the other hand, often slow in response time.
In miniaturization of switches it is a desirable goal to eliminate as many mechanically interrelated parts, to simplify manufacturing costs, and, at the same time, maintain a high degree of reliability. One frequently tried approach is to employ a conductive fluid, such as mercury, as the contact or "throw" of the switch. One basis for employing mercury is that it will not suffer the wear and failure of "throws" made of solids. Such solid "throws" are mechanically linked to switch contacts and are the first parts of such switches to experience wear. Thus, an electrically conductive fluid used in place of a mechanical throw clearly improves the life of a switch. However, such switches are not believed to be capable of fast operation. Most importantly, such switches are dependent upon their position with respect to ground since the mercury throw position or resting place is dependent on gravity.
Still another problem in the use of a switch is encountered when it is employed as a sensing device. Where switch contact elements (such as push buttons or lever arms) are used to directly sense the holes punched in, for example, a hollerith card or the embossments on a credit card, difficulty is encountered. The requirements of spacing make it hard to fit such switches into a confined space. Dust and dirt very often play a material part in the life of such switches.
With particular reference to the credit card industry, the importance of small, accurate, and long-lasting switches becomes immediately apparent. One suggested means for reading the embossments on credit cards, for example, comprises a series of seven fingers appropriately placed and secured to L-shaped pivot members. As each finger is depressed, the one arm of the L-shaped member to which it is attached pivots outwardly. A magnetized tooth is affixed to the other arm of the L. This pivotal placement requires a second step. A magnetic head proceeds past the teeth and "reads" their position thus determining the symbols "read" by the fingers. This system therefore requires mechanical interlinkage and the capability of magnetically reading the teeth. Thus, there is provided a mechanically cumbersome and intricate device for reading embossed or punched indicia. Embossments must be read mechanically. The mechanical reading must then be read magnetically and finally translated into electrical signals.
Still another device employs optical reading of punched or embossed indicia. This is believed to be a relatively expensive arrangement. The expense may well be prohibitive where, for example, it is desirable to put a credit card reader into the hands of small merchants or large stores with numerous reading stations.
Optical sensors must first provide mechanical means for interrupting light transmitting means and then convert the light patterns produced thereby into electrical signals. Because of the expense of individual sensors, it is usually required that each character be read one at a time, a time-consuming activity. In addition, dust may block holes thereby giving a false reading.
The sensitivity of a switch employed to read, for example, embossed letters and numbers on credit cards may be appreciated by the dimensions suggested by the American National Standards Specifications for Credit Cards, ANSI X4,13-1971 that suggest that embossed letters and numbers have a height above the card of 0.019 +0.000 and -0.002 of an inch and be OCR Size C or Farrington 7B type style. Thus, if a seven point measuring array is used, such seven switches must each occupy an area of approximately 1.35 of an inch by 0.196 of an inch.
Reading such characters directly by a switch offers the difficulty of fitting such switches into a very small space.
A number of switches have been devised employing mercury as a "throw". Few, however, are so designed as to be employed for miniaturized use for reading credit card numbers, rapid reading, or for continuous use in an environment of dust or dirt.
One example of a suggested mercury switch is provided by Charles in U.S. Pat. No. 1,806,236. That patent disclosed a plunger floating on a pool of mercury in a cavity formed in a housing. The plunger is provided with a plurality of channels which, in conjunction with the side wall of the housing forms passageways. When the plunger is depressed, mercury is guided up the passageways and into contact with electrical contacts in the wall of the housing. However, Charles employed this switch so that mercury would flow freely. The main object of the switch was to permit the mercury to flow out of the cavity and into contact with shorting contacts. The result of such an arrangement was that if the switch were installed in an automobile and that automobile turned over, the switch would spill its mercury thereby shorting the switch and turning off the devices to which it was coupled, such as a motor.
Another suggested device is provided by Lanctot in U.S. Pat. No. 3,184,693. Lanctot proposes a circular recessed body member for holding a quantity of mercury. Disposed over the entire area is a flexible diaphragm. Pressure upon the diaphragm is used to distort the mercury to couple electrical contacts at the periphery of the body member. This device suffers from a number of disadvantages. For example, the device may be position sensitive. A sudden jolt to the switch can dislodge the mercury to one side, destroying the switch operation. Further, depression of the diaphragm must necessarily require any gas (such as air) in the area to be displaced. Atmospheric conditions may cause the diaphragm to become distorted, further distorting the operational characteristics of the swtich. Lanctot, however, discloses no means for removal of the gases displaced by the mercury and diaphragm. If, on the other hand, the mercury were placed in a vacuum, the diaphragm would necessarily be pulled inward making the device inoperative. Still another problem with such a device is that mercury abhors corners. That is, the mercury will tend to withdraw from the edges and thereby resist being pushed toward the electrical contacts. Thus, the "making" of such a switch may be extremely unreliable. Further, the use of a flexible diaphragm provides a moving part which may be subject to permanent deformation or wear.
Still another device has been suggested by Schmid in U.S. Pat. No. 3,358,109. Schmid proposes a plate with pools of mercury disposed in recesses formed therein. The plate is made of an insulating material and the recesses comprise the ends of electrical contacts. An actuating plate of insulating material is employed to press down upon the mercury pools, causing them to flow and join thereby "making" the switch. Schmid, however, uses the two pools of mercury as extensions of the contacts. The amounts of mercury at each location is clearly variable, and a function of the amount each pool pulls to itself when the actuating plate is raised. Thus, the operating characteristics of the switch may vary considerably. Furthermore, the flowing mercury, and closing of the space between the plate and actuating plate, requires that the gases trapped within the space must be displaced. Schmid specifies that the switch he proposes be hermetically sealed and shows no means of removing the displaced gases. Thus, gases may be forced into the pools, distorting their operation and blocking electrical contact between pools.